Many buildings are designed to be as airtight as possible in order to save energy that would otherwise be expended on heating and cooling. It has been discovered, however, that such buildings can suffer from an excessive build up of indoor air contaminants from a variety of sources. A possible solution to indoor air quality issues is to ventilate.
A number of studies have indicated that buildings should be ventilated with specific amounts of outside air to counter this potential build up of indoor air contaminants. Consequently, the American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) has established indoor air standards that have been adopted by many building codes and design engineers. In particular, ASHRAE standards dictate that buildings should be ventilated with differing amounts of outside air based on a number of factors such as potential pollutant levels and occupancy levels.
A number of strategies have previously been provided in answering the need for ventilation. One solution is a fixed air strategy in which a building's HVAC system permits a fixed amount of outside air to enter the building at all times through either passive or active ventilation. While this solution can provide ample fresh air, energy may be wasted heating or cooling fresh air during times when the building is unoccupied or only lightly occupied.
Another strategy is to base ventilation rates on estimated, or projected, occupancy. In particular, unnecessary ventilation may be reduced by estimating or guessing times when a building or a portion of a building such as a room will be less heavily occupied and therefore providing a reduced level of ventilation at such times. This strategy may require manual adjustments of ventilation levels in response to varying occupancy estimates and furthermore may result in incorrect levels of ventilation when occupancy estimates are in error.
Many HVAC systems now include energy recovery ventilators, or air-to-air heat exchangers, in order to capture some of the thermal energy that would otherwise be lost due to exchange of stale indoor air for fresh outdoor air. For example, during a cooling system, the exiting stale (but still cool) air is used to remove at least some heat from the incoming fresh (but relatively warmer) air. During a heating season, the exiting (but still warm) air is used to add at least some thermal energy to the incoming fresh (but relatively colder) air. In some instances, these energy recovery ventilators operate constantly and therefore add load to the HVAC system by over ventilating when ventilation needs are reduced.
A useful strategy is known as demand ventilation, in which the amount of fresh air provided to a building or a portion of a building is varied to accommodate actual demand. While this strategy can address many of the shortcomings of other ventilation schemes such as those discussed above, the strategy inherently relies upon some reliable indication of building occupancy levels.
In some cases, building occupancy levels can be indicated by a variety of different sensors. Unfortunately, sensors can fail. In some systems, a failed sensor or a reading that indicates a failed sensor causes the ventilation system to default to a full ventilation mode in which the ventilation damper or dampers revert to a fully open position.
While this strategy ensures at least adequate ventilation, this can result in excessive ventilation, thereby wasting energy that has been used to heat or cool the air within the building. A need remains, therefore, for demand ventilation protocols that address the issue of sensor failure while still providing for desirable levels of energy conservation.